Methods for manufacturing polymer particles largely include emulsion polymerization, dispersion polymerization, seed polymerization and suspension polymerization. Among them, the emulsion polymerization method is employed the most widely because it is easy to manufacture particles of a very uniform particle size distribution. However, the polymer particles manufactured by emulsion polymerization do not exceed 1 μm in diameter and a surfactant used to improve the stability of the particles tend to be adsorbed on the surface of the particles, thereby causing foaming or deterioration of physical properties.
In dispersion polymerization, ethanol, methanol, etc. may be used alone as a reaction medium, or another organic solvent such as toluene, benzene, 2-methoxyethanol, etc. or a small amount of water may be used together as a cosolvent. The size of the polymer particles manufactured by dispersion polymerization using such media is typically 1 μm or smaller. In addition, the dispersion polymerization method is disadvantageous in that particle size distribution varies very sensitively depending on the reaction environments such as the composition of reactants, presence of oxygen, etc. and process reproducibility is not good.
Seed polymerization is a method of manufacturing polymer particles with uniform particle size distribution of micrometer scales through swelling of monomers after dispersing particles of uniform size prepared through emulsion polymerization or dispersion polymerization in a dispersion medium. Although control of particle size is easy in seed polymerization, it is disadvantageous in that the polymerization process is very complicated and a long time is required because the polymerization is conducted in two or three steps.
Suspension polymerization is a method of manufacturing polymer particles from water-insoluble monomers using water as a dispersion medium in the presence of a steric stabilizer. However, because this method manufactures polymer particles by dispersing monomers in an aqueous solution by applying mechanical force, the obtained polymer particles have a very broad particle size distribution ranging from 0.1 to 1000 μm and an additional apparatus is necessary to reduce the particle size distribution. U.S. Pat. Nos. 4,017,670, 4,071,670, 4,085,169 and 4,129,706 and European Patent No. 0,443,609 disclose methods for manufacturing polymer particles with a final particle size of 5-50 μm by conducting suspension polymerization at a very high stirring speed of 10,000-30,000 rpm using three reactors connected serially or in parallel. U.S. Pat. No. 5,852,140 disclose a method for manufacturing polymer particles with a particle size of 0.1-5 μm by conducting bulk polymerization of monomers until a conversion rate reaches about 50% and then conducting second polymerization after dispersing the resulting oligomers in an aqueous solution in the presence of a dispersion medium using a high shear mixer.
However, because it is difficult to manufacture polymer particles with a uniform particle size through the suspension polymerization, Japanese Patent Publication No. H11-60615 and International Patent Publication No. WO99/19370 disclose a method of forming an aqueous solution of monomer droplets of relatively uniform particle size in the presence of a dispersing agent by employing the SPG (Shirasu porous glass) membrane emulsification technique and then producing polymer particles with a uniform diameter of 1-10 μm through suspension polymerization. However, this method requires an additional process of the membrane emulsification and is uneconomical because the expensive solvent 1,1,1,3,3,3-hexafluoro-2-propanol is used to manufacture polydioxanone (PDO) particles.